My car has factory power front disks and rear drums. When I bought it, it had set for several years, and the brakes were toast. I installed a new master cylinder, booster, lines and hoses, rear wheel cylinders and I rebuilt the distribution block and proportioning valve. Pads front and rear had a lot of meat left, so I reused them as well as the disks/drums.
After the rework, the brakes were operable but weak, so I jumped back in to try to find the problem. Performing the skid test, I could only get the rear brakes to lock up. I suspected (pretty certain) that I had damaged the check valve in the proportioning valve, so I started by swapping in a Wilwood adjustable proportioning valve. I went through the set-up/adjustment procedure per Wilwood instructions, and it didnāt make a difference. Even with the valve adjusted all the way in (total bias to the front brakes) I could still only get the rears to lock up. It essentially feels exactly like it did before the PV swap.
Last night, I pulled the calipers off and verified that they move when the brake pedal is compressed, but I think maybe the shuttle in the distribution block has moved? After installing the new proportioning valve, I reset the shuttle and bled the system with a vacuum bleeder. In theory it shouldnāt have moved, correct? I should also mention that my brake pressure switch is stuck in the āOnā position, so unless I unplug it, the light in the dash is always on. I have a new one on order.
My plan of attack is to pull the distribution block/PV assembly, recenter the shuttle, lock it in place and test again. The reason Iām reaching out is that R&R of the distribution block is such a PITA! I wanted to see if there are suggestions of anything else I should look at first. Any help is appreciated.
Itās unlikely the distribution block is an issue. It functions as a place to mount the switch which operates an idiot light. It has no other function.
I was under the impression that in the event of a brake line rupture, the shuttle in the distribution block would move and block off the offending system so you wouldnāt lose all brake function. Maybe thatās not true with the divided reservoir master cylinder?
If youāre correct, then Iām suspicious of the front pads. Maybe theyāre rock hard and not providing any friction? That would be an easy fix.
I havenāt done anything with caliper location since Iāve owned the car, but I believe theyāre installed correctly. Bleeders point rearward and are on the tops of the calipers.
The calipers are suspect since sitting so long. Even though they move, they may have a rust ridge internal that stops them b4 they actually press the pads enough to stop. Had it happen to rebuilt ones that sat on shelf 4 a few yearsā¦
The 68-73 calipers are side-specific. If you had them off and mixed them up, you will never get the air out. Be sure your bleeders are pointing to the rear of the car. If they are pointing upwards, theyāre on the wrong side and need to be swapped.
I agree with PCM that both calipers having issues at the same time seems unlikely, but the car did sit for ~8 years before I picked it up. Weāll see.
I went ahead and bought rebuilt calipers. They donāt cost much more than rebuild kits, and I was afraid Iād find cratered pistons once I tore into my old ones. I got them installed last night and will bleed tonight. Itās supposed to rain tomorrow, so I may not get a test drive in for awhile, but Iāll let you know how it goes.
Ford refers to it as the āPressure Differential Valveā. It actually performs two functions, one being to block off the circuit with the leak, the other being that when the valve moves off center it turns on the light.
Are you power brakes? If yes, how old is your booster? They unfortunately are suspect after perhaps 10 years or so - used alot or not.
Yes the calipers can seize (Ford specād out a tighter clearance on the early Kelsey Hayes than on the Corvettes). I believe those were changed sometime not long after '67.
If you replaced your booster and itās a Cardone - they are total crap. I hear other brands are dubious at best as well. That happened to me the last time mine died. It was under lifetime warranty (Advance Auto) - they were annoyed because they had to pay to ship it off and get it rebuilt on their nickel. I put it on and still had issues. Thinking there was no way the ānewā booster was bad, I chased my tail. after some time with a bud, we concluded it must have been the new booster bad. I bought a different booster and voila - problem solved. The new POS rebuild by Cardone was junk. I actually then got Advance Auto to reimburse me for the replacement (that was a challenge - helps to know the right folks).
Jumping in late here, but how about looking into the brake lines, especially the flexible ones. Im also on the booster camp also,. I rebuilt the distribution block replaced the flex lines,the brake booster and calipers before i was happy
New calipers are on, brakes are bled and the weatherās been bad so I havenāt had a chance to test it. Maybe tomorrow. My new brake pressure switch is supposed to arrive today, so Iāll get that installed before I take it out. That should give me reliable information on pressure differential valve position.
To answer some of the follow up questions and suggestions, I replaced the booster and hoses during the initial ārepairā. I donāt recall the booster brand. Iād have to dig through a pile of receipts, but the power function works fine. I also verified the hoses are clear (removed them and blew though them) while I had the calipers off.
My issue is that the front brakes are either extremely week or nonexistent. Rear brakes are fine and are doing the bulk of the work. Thanks again for all the input. Iāll get it sorted eventually.
As Royce has noted, your pressure differential valve position should only affect whether the brakes warning light is on or not. If that valve sealed off leaky front or back brakes, the master cylinder could no longer work because of the trapped brake fluid between master cylinder and closed pressure differential valve. Good luck finding your problem.
This might be kind of obvious, but the rotors must be completely free of any oil or grease. You need to spray them down front and back with brake parts cleaner.
